This invention relates broadly to an improvement in nuclear fuel elements for use in the core of nuclear fission reactors, and more particularly to improved nuclear fuel elements having a refractory metal liner incorporated in the fuel element between the cladding and the nuclear fuel.
Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which can have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, non-reactive, heat conductive container or cladding. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a selfsustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The cladding serves two primary purposes: first, to prevent contact and chemical reactions between the nuclear fuel and the coolant or the moderator if a moderator is present, or both if both the coolant and the moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator or both if both the coolant and moderator are present. Common cladding materials are stainless steel, aluminum and its alloys, zirconium and its alloys, niobium (columbium), certain magnesium alloys, and others. The failure of the cladding, i.e. a loss of the leak tightness, can contaminate the coolant or moderator and the associated systems with radioactive long-lived products to a degree which interferes with plant operation.
Problems have been encountered in the manufacture and in the operation of nuclear fuel elements which employ certain metals and alloys as the clad material due to mechanical or chemical reactions of these cladding materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent materials as a nuclear fuel cladding since they have low neutron absorption cross sections and at temperatures below about 600.degree. F are strong, ductile, extremely stable and non-reactive in the presence of demineralized water or steam which are commonly used as reactor coolants and moderators. Within the confines of a sealed fuel element, however, the hydrogen gas generated by the slow reaction between the cladding and residual water inside the cladding may build up to levels which under certain conditions can result in localized hydriding of the alloy with concurrent local deterioration in the mechanical properties of the cladding. The cladding is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide over a wide range of temperatures. Also, fuel element performance has revealed a problem with splitting of the cladding due to interactions between the nuclear fuel, the cladding and the fission products produced during nuclear fission reactions.
The zirconium cladding of a nuclear fuel element is exposed to one or more of the gases listed above and fission products during irradiation in a nuclear reactor and this occurs in spite of the fact that these gases may not be present in the reactor coolant or moderator, and further may have been excluded as far as possible from ambient atmosphere during manufacture of the cladding and the fuel element. Sintered refractory and ceramic compositions, such as uranium dioxide and other compositions used as nuclear fuel, release measurable quantities of the aforementioned gases upon heating, such as during fuel element manufacture and especially during irradiation. Particulate refractory and ceramic compositions, such as uranium dioxide powder and other powders used as nuclear fuel, have been known to release even larger quantities of the aforementioned gases during irradiation.. These released gases are capable of reacting with the zirconium cladding containing the nuclear fuel. This reaction can result in the embrittlement of the cladding which endangers the integrity of the fuel element. Although water and water vapor may not react directly to produce this result, at high temperatures water vapor does react with zirconium and zirconium alloys to produce hydrogen and this gas further reacts locally with the zirconium and zirconium alloys to cause embrittlement. Release of these residual gases within the sealed metal-clad fuel element also increases the internal pressure within the element and thus introduces additional stresses in the presence of corrosive conditions. Only recently has it been discovered that these undesirable results are exaggerated by the localized mechanical stresses due to fuel-cladding differential expansion (localized stress at UO.sub.2 cracks). Corrosive gases are released from the cracks in the fuel at the very point of localized stress at the intersection of the fuel cracks with the cladding surface. The localized stress is exaggerated by high friction between the fuel and the cladding.
Thus in light of the foregoing, it has been found desirable to minimize attack of the cladding from water, water vapor and other gases, especially hydrogen, reactive with the cladding inside the fuel element throughout the time the fuel element is used in the operation of nuclear powder plants. One such approach has been to find material which will chemically react rapidly with the water, water vapor and other gases to eliminate these from the interior of the cladding, and such materials are called getters.
Another approach has been to coat the nuclear fuel material with a ceramic to prevent moisture coming in contact with the nuclear fuel material as disclosed in U.S. Pat. No. 3,108,936. U.S. Pat. No. 3,085,059 presents a fuel element including a metal casing containing one or more pellets of fissionable ceramic material and a layer of vitreous material between the ceramic material and the casing, and the vitreous material is bonded to the ceramic pellets. This assures uniformly good heat conduction from the pellets to the casing. U.S. Pat. No. 2,873,238 presents jacketed fissionable slugs of uranium canned in a metal case in which the protective jackets or coverings for the slugs are a zinc-aluminum bonding layer. U.S. Pat. No. 2,849,387 discloses a jacketed fissionable body comprising a plurality of open-ended jacketed body sections of nuclear fuel which have been dipped into a molten bath of a bonding material giving an effective thermally conductive bond between the uranium body sections and the container (or cladding). The coating is disclosed as any metal alloy having good thermal conduction properties with examples including aluminum-silicon and zinc-aluminum alloys. Japanese Patent Publication No. SHO 47 -46559 discloses consolidating discrete nuclear fuel particles into a carbon-containing matrix fuel composite by coating the fuel particles with a high density, smooth carbon-containing coating around the pellets. Still another coating disclosure is Japanese Patent Publication No. SHO 47-14200 in which the coating of one of two groups of pellets is with a layer of silicon carbide and the other group is coated with a layer of pyrocarbon or metal carbide.
The coating of nuclear fuel material introduces reliability problems in that failure to achieve uniform coatings free of faults is difficult. Further, the deterioration of the coating can introduce problems with the long-lived performance of the nuclear fuel material.
Another approach has been to introduce a barrier or matal liner between the nuclear fuel material and the cladding holding the nuclear fuel material as disclosed in U.S. Pat. No. 3,230,150 (copper foil), German Patent Publication DAS No. 1,238,115 (titanium layer), U.S. Pat. No. 3,212,988 (sheath of zirconium, aluminum or beryllium), U.S. Pat. No. 3,018,238 (barrier of crystalline carbon between the UO.sub.2 and the zirconium cladding), and U.S. Pat. No. 3,088,893 (stainless steel foil). While the barrier concept proves promising, several of the foregoing references involve incompatible materials with either the nuclear fuel (e.g. carbon can combine with the oxygen from the nuclear fuel), or the cladding (e.g. copper and other metals or carbon can diffuse into the cladding, altering the properties of the cladding), or the nuclear fission reaction (e.g. by acting as neutron absorbers).
The foregoing discussion of the prior art demonstrates that there exists a need for a fuel element having a gap between the nuclear fuel and the cladding which incorporates protection (1) inhibiting the mechanical interaction between the cladding and the nuclear fuel due to swelling or expansion of the fuel during operation, (2) isolates fission products produced in the fuel from the cladding and (3) improves the axial thermal peaking gradients along the length of the fuel rod.
Accordingly, it has remained desirable to develop nuclear fuel elements minimizing the problems discussed above.